Core-shell particles with catalytic activity

ABSTRACT

The present invention pertains to novel core-shell particles comprising a core of alumina and a shell of cobalt oxide, characterized in that they are spherical with a number average diameter, as measured by TEM, of between 10 and 30 nm. This invention also pertains to the method for preparing these core-shell particles and to their uses in the manufacture of a catalyst.

RELATED APPLICATIONS

The present application is a National Phase entry of PCT Application No.PCTEP2013002627, filed Dec. 4, 2013, which claims priority from EPPatent Application No. 12306513.8, filed Dec. 4, 2012, said applicationsbeing hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention pertains to novel core-shell particles comprisinga core of alumina and a shell of cobalt oxide, characterized in thatthese particles are spherical with a number average diameter, asmeasured by TEM, of between 10 and 30 nm. This invention also pertainsto the method for preparing these core-shell particles and to their usesin the manufacture of a catalyst.

BACKGROUND OF THE INVENTION

The Fischer-Tropsch process generally comprises a first step whichconsists in reacting a source of carbon (such as coal, natural gas orbiomass) with a source of oxygen (such as steam, air or oxygen) to forma mixture of carbon monoxide and hydrogen, usually referred to assyngas. A second step is then carried out, which involves contacting thesyngas with a Fischer-Tropsch catalyst, which leads to hydrocarbons andwater. The main products of the Fischer-Tropsch reaction are linearolefins and paraffins and water. It is well known that the nature of thehydrocarbons produced, and their chain length, may vary depending on theprocess conditions and the catalyst used. The third step involvesisomerisation of the hydrocarbons formed in the second step to producemore valuable products. For instance, the longer chains in the productmay be cracked to form products in the diesel or gasoline range, andlinear paraffins may be isomerised to improve diesel product propertiessuch as cloud point and pour point. Generally, adapted hydrotreatingcatalysts are used for this third step.

Typical catalysts used in the second step of the above process are madeby kneading or impregnation of carriers made of micrometric particles ofalumina, silica, titania, silicon carbide, carbon or mixtures thereof,with a metal such as cobalt having a particle size comprised between 10and 20 nm.

They suffer from the drawback that most of the catalytic metal, which isinside of the catalyst particles, does not participate in the catalyticprocess. There is thus a waste of the catalytic metal which iseconomically disadvantageous, especially in the case of cobalt, which isa rather expensive catalytic metal. In order to overcome this drawback,the present inventors have contemplated using core-shell particles,which comprise a core in a cheaper material than cobalt and a shell ofcobalt. These particles comprise a much lower amount of cobalt thanknown catalysts, while approaching the same catalytic activity.

A cobalt-based catalyst, comprising particles having a core-shellstructure and which is said to be selective towards the Fischer-Tropschreaction, has been disclosed in U.S. Pat. No. 7,361,626. This catalystmay be prepared according to the following process. A zinc oxide layeris first applied on the surface of an oxidic core material, typically,alumina, by means of a so called “layer-by-layer” (or “LBL”) method, soas to obtain a core-shell support. A catalytically active material, suchas cobalt, is then added to this core-shell support either byimpregnation or deposition-precipitation. Thereafter, the resultingparticles are calcined and hydrogenated to produce a metal-basedcatalyst. This process involves several steps and the use of asurfactant to anchor the zinc oxide layer to the chemically inertaluminium core, which increases its cost. Moreover, the core-shellparticles thus obtained can be described as having a core-shell supportcoated with large crystallites of cobalt. Consequently, they do notallow reducing the cobalt content of the catalyst particles, which isnot the purpose for which these attrition-resistant particles have beendesigned. Moreover, the size of these particles may be detrimental tothe activity and selectivity of the catalyst made therefrom.

Therefore, there remains the need to provide a simple and cost-effectivemethod for preparing core-shell particles which dimensions may be easilycontrolled and which may be used to manufacture a catalyst having goodselectivity towards the Fischer-Tropsch reaction and good productivity.

This need has been satisfied by a novel method which involveshomogeneous deposition-precipitation of cobalt carbonate ontonanoparticles of alumina. This method leads to specific nanoparticleshaving a core consisting of nanoparticles of alumina and a shellcomprising cobalt. To the inventors' knowledge, these particles havenever been described before.

The precipitation-deposition method has already been applied in U.S.Pat. No. 7,851,404 and US 2007270514 to the manufacture of cobalt-basedcatalysts. In these documents, a cobalt compound, obtained bydecomposition of a cobalt amine complex under basic conditions, isdeposited onto particles of a carrier material in the form of a powderor of a shaped granular material, or onto a titania-coated alumina. Thecarrier core has a mean diameter of several microns and no informationis given about cobalt crystallite size. The thickness of the cobaltlayer in US 2007270514 ranges from 5 to 250 μm. It has been shown in theExamples below that the productivity, on cobalt mass basis, of thesecatalysts could still be improved.

Other core-shell nanoparticles comprising a core of a carrier materialselected from iron oxide, copper and silicon dioxide have been disclosedin EP 2 530 125, Nachal D. Subramanian et al., Catalysis Science &Technology, Vol. 2, No. 3, January 2012 and in CN 101 954 256,respectively. In the second one of these documents, the core-shellparticles need to be oxidized so as to remove the surfactants used intheir synthesis and bound to the surface of the nanoparticles. Theresulting nanoparticles show a polyhedron-like morphology with somediffusion of copper from the core to the surface of the nanoparticles.

SUMMARY OF THE INVENTION

In one aspect, the present invention is thus directed to a method forthe preparation of spherical core-shell particles, comprising thesuccessive steps consisting of:

(a) mixing a cobalt salt with (i) ammonium carbonate, bicarbonate orcarbamate and (ii) ammonium hydroxide in water, so as to obtain anaqueous solution comprising a cobalt amine complex;(b) heating said aqueous solution to a temperature comprised between 40and 90° C., either before, while or after adding particles of aluminahaving a primary particle size of less than 100 nm, so as to precipitatecobalt carbonate onto the surface of the alumina primary particles andto obtain core-shell particles having a core of alumina and a shell ofcobalt carbonate;(c) optionally recovering, washing and/or drying said precipitate;(d) converting cobalt carbonate in the shell of said core-shellparticles into cobalt oxide.

It is understood that the above method may comprise other preliminary,intermediate or subsequent steps, as long as they do not impair thestructure and properties of the core-shell particles obtained.

In another aspect, this invention pertains to the core-shell particleswhich may be obtained according to the above method, comprising a coreof alumina and a shell of cobalt oxide, characterized in that they arespherical with a number average diameter of between 10 and 30 nm andpreferably between 10 and 20 nm.

It should be noted that all the particle sizes referred to in thisspecification are measured by TEM (Transmission Electron Microscopy).

In still another aspect, this invention pertains to a catalystcomprising core-shell particles as defined above, embedded in a carriercomprising nanometric particles of a carrier material.

The expression “nanometric particles” refers to particles having anumber average diameter, as measured by TEM, of below 100 nm, preferablybelow 30 nm, more preferably below 20 nm and typically between 5 and 15nm.

In yet another aspect, this invention pertains to the use of the abovecore-shell particles for manufacturing an activated catalyst, accordingto the following successive steps:

mixing the core-shell nanoparticles of this invention with nanometricparticles of a carrier material and water so as to obtain a slurry,

homogenizing and drying said slurry to obtain a porous catalyst andoptionally shaping said catalyst,

reducing said catalyst in order to at least partially convert cobaltoxide into elemental cobalt.

In still another aspect, this invention pertains to the uses of saidactivated catalyst.

The method of this invention allows preparing size-tailored core-shellnanoparticles under economical conditions, especially because itcomprises only a few synthetic steps which all use standard low costchemicals and do not generate hazardous by-products or contaminatedwaters that should be further treated. This method results innanoparticles provided with an outer cobalt layer which is only a fewnanometers thick. Therefore, most of the cobalt contained in theseparticles is surface cobalt, which is entirely involved in the catalyticreaction.

DETAILED DESCRIPTION OF THE INVENTION

This invention will now be described in further details. In thefollowing description, the expression “comprised between” should beunderstood to designate the range of values identified, including thelower and upper bounds.

The novel method of this invention for the preparation of core-shellparticles mainly involves the precipitation of a cobalt carbonate shellaround a core consisting of nanometric particles of alumina.

Specifically, in the first step of this method, a cobalt salt is mixedwith ammonium carbonate, bicarbonate or carbamate and with ammoniumhydroxide in water, so as to obtain an aqueous solution comprising acobalt amine complex, [Co(NH₃)₆]²⁺. Among the cobalt salts, anyinorganic salts may be used, but salts of divalent anions such ascarbonate are preferred. This step can be carried out at a temperatureof 15 to 30° C., preferably from 20 to 25° C., by simply dissolving,generally under stirring, the cobalt salt in a solution of ammoniumcarbonate, bicarbonate or carbamate in aqueous ammonium hydroxide. Theamount of these reactants should preferably be such that the pH of thismixture is between 7.5 and 10, preferably between 9 and 10. The cobaltsalt may represent from 0.5 to 10 wt. %, preferably from 1 to 8 wt. %,and more preferably from 3 to 7 wt. %, of the solution. The solutionthus obtained preferably contains from 0.1 to 2.5 moles of the cobaltamine complex per litre.

This solution may be filtered so as to remove any possible residue ofcobalt salt that would not have dissolved.

This solution is then heated at a temperature comprised between 40 and90°, preferably between 50 and 70° C. Alumina comprising nanoparticleshaving a substantially spherical shape and a primary particle size ofless than 100 nm, preferably less than 30 nm and more preferably lessthan 20 nm, typically between 5 and 15 nm, are added to this solutionduring, after, or preferably before, the heating step. These particlesmay have a surface area of about 80 to 120 m²g. They may be used in theform of aggregates which will readily disintegrate into primaryparticles in an aqueous solution under agitation, such as the productsold by EVONIK under the trade name Aeroxide® Alu-C. The addition ofthese particles is usually carried out while agitating the solution, forinstance by mechanical stirring. In this step of the method, at leastone other metal salt may be added, especially salts of catalyticpromoters such as platinum, manganese or ruthenium and their mixtures.

The atomic ratio of alumina to cobalt may be tuned as needed and may forinstance range from 100:1 to 1:3 and preferably from 1:1 to 1:2. Theslurry formed may be maintained at the aforesaid temperature for 2 to 72hours, preferably from 12 to 36 hours. Such heating results in theevaporation of ammonia and carbon dioxide and thus in a lowering of thepH, which itself gives rise to the decomposition of the cobalt aminecomplex into cobalt carbonate. The latter precipitates onto the surfaceof the alumina particles which thus act as a nucleating agent.

Although not necessary, this precipitate may then be recovered by anyappropriate means, such as by filtration, centrifugation or any othermethod for separating solids from liquids. It is then preferably washedwith water at 10-40° C., for instance at 10-30° C. It may then be dried,for instance at a temperature between 100 and 150° C.

Core-shell particles having a core of alumina and a shell of cobaltcarbonate are thus obtained.

These core-shell particles are then treated so as to convert cobaltcarbonate into cobalt oxide. This can be achieved by prolonged heatingat the precipitation temperature, i.e. between 50 and 90° C. orpreferably between 50 and 70° C., or by calcination at a temperature of250 to 600° C., and preferably from 300 to 500° C.,

The oxidic core-shell particles thus obtained have a spherical shapewith a number average diameter, as measured by Transmission ElectronMicroscopy (TEM), comprised between 10 and 30 nm and preferably between10 and 20 nm. The thickness of the cobalt oxide shell is comprisedbetween 1 and 5 nm and preferably between 1 and 3 nm, as also measuredby TEM. The lower this thickness, the higher the cost savings comparedto usual cobalt particles used in catalysts. Usually, the particles ofthis invention do not include any other metal oxide then cobalt oxideand the alumina core, except the above-mentioned promoters, if present.In case these promoters are needed, they can be added to thenanoparticles of this invention either during their synthesis, asexplained above, or after their synthesis, for instance by impregnation.

These cobalt oxide alumina nanoparticles constitute a catalystprecursor, in that they may be used to manufacture catalysts involved inhydrogenation reactions such as the hydrogenation of aromatic orolefinic compounds, e.g. waxes, nitro, nitrile or carbonyl compounds,such as the conversion of nitrobenzene to aniline or the conversion offatty nitriles to amines or the hydrogenation of aldehydes to thecorresponding alcohols. They may also be useful in various otherreactions and more specifically in the Fischer-Tropsch process. This maybe part of an overall process for the conversion of natural gas topetroleum compounds wherein the hydrogen/carbon monoxide gas mixtureused in the Fischer-Tropsch reaction is a syngas formed by steamreforming natural gas.

Whatever the reaction in which they are used, the nanoparticles of thisinvention are preferably included within a carrier which comprises from50 to 100%, and preferably from 90 to 100% of nanometric particles of acarrier material, relative to the total weight of the carrier, so as toform a catalyst. This carrier material may be the same as that used inthe core of the particles, i.e. alumina, or it may be different. In thelatter case, it may be selected from the group consisting of silica;titania; activated carbon; silicon carbide; and their mixtures.Preferably, the same material is used both in the core of the particlesand as a carrier in this step of the process. This carrier is usuallybeneficial to the retention of the core-shell structure under reducingconditions. Moreover, it has been shown that this specific carrieravoided deactivation of the catalyst by sintering, which usually occurswhen impregnating core-shell nanoparticles into micrometric carrierswhich bind weakly to these nanoparticles. Sintering may thus be reducedby mixing the core-shell nanoparticles with the nanometric particles ofthe carrier material, preferably in the presence of water, so as toobtain a slurry which is then homogenized by stirring. This ensures ahigh dispersion of cobalt in the catalyst prepared from this slurry. Theoptimum amount of oxidic core-shell particles present in the carrier mayvary, depending on the catalytic activity required. Typically, theamount of cobalt present in the catalyst may range from 1 to 25% byweight of catalyst, for instance from 10 to 20% by weight of catalyst.The amount of cobalt may be easily tuned by adding an appropriate amountof core-shell particles to the nanometric particles of the carriermaterial, without altering the particle size and the level ofdispersion.

The above slurry is further dried, for instance at a temperature between30 and 90° C., which results in a porous catalyst, usually a mesoporouscatalyst, in the form of a powder, having oxidic core-shellnanoparticles embedded within its structure. This catalyst may be shapedor formed by means of spray drying, pelletizing, (wheel) pressing,extrusion, or application on a metal support like a metal wire. It ispreferably shaped into pellets.

After optional shaping, the catalyst is generally activated, i.e.reduced by contacting it with hydrogen, optionally diluted with an inertgas such as nitrogen, typically at temperatures of about 300° C. to 800°C., preferably between 300 and 400° C., in order to convert cobalt oxideinto elemental cobalt. Preferably, at least 70 wt. % of the total cobaltin the activated catalyst will be in the elemental state.

The activated catalyst may then be used as a slurry catalyst orpreferably as a fixed bed catalyst. For instance, if developed forcarrying out the Fischer-Tropsch reaction, this catalyst may be used infixed bed reactors, especially multi-tubular fixed bed reactors,fluidised bed reactors, such as entrained fluidised bed reactors andfixed fluidised bed reactors, and slurry bed reactors such asthree-phase slurry bubble columns.

This invention will be better understood in light of the followingexamples which are given for illustrative purposes only and do notintend to restrict the scope of this invention as defined by theattached claims.

EXAMPLES Example 1: Synthesis and Characterization of CoAl₂O₃ Core-ShellParticles

77.3 mL of demineralised water and 5.47 g of ammonium carbonate wereadded to 30.5 mL of a 25 wt. % ammonia aqueous solution. The solutionwas stirred while 0.74 g of cobalt carbonate was added. After thesolution turned dark red, it was filtered and 0.18 g of aluminaparticles (Aeroxide Alu-C supplied by ALDRICH, diameter=13 nm, specificarea=100 m²g) was then added to the filtrate containing the cobalt aminecomplex thus formed. The reaction mixture was heated from roomtemperature to 70° C., using an oil bath, and the temperature wasmaintained during the precipitation process. A suspension of core-shellparticles was thus obtained. The particles suspension was further heatedat 70° C. so as to obtain core-shell particles with a core of aluminaand a shell of cobalt oxide, consisting mainly of Co₃O₄, with athickness of about 3 nm.

0.93 g of these particles was then added to 1.65 g of the same aluminaas used for the core. A slurry was prepared from this mixture by adding100% of water relative to solid material. The slurry was then dried andshaped in order to produce pellets of a catalyst precursor.

The catalyst thus obtained contained 20 wt. % of cobalt, relative to thetotal weight of the catalyst. EDX line analysis revealed the core-shellstructure of the particles. FIR-TEM characterization of these particlesdisplayed alumina particles decorated with 3 nm sized cobalt oxidenanoparticles so as to form a sort of “berry”.

Example 2: Catalytic Tests

The catalyst of Example 1 was reduced by heating the reactor to 435° C.in H₂ (20 l/h STP, 3 Kmin). The temperature was maintained for 10 h. Thereduced catalyst was cooled to room temperature in hydrogen.Fischer-Tropsch synthesis was conducted by heating the reactor to 210°C. at 30 bars for approximately 5 h in synthesis gas at approximately10% conversion and holding the reactor in those conditions forapproximately 100 h, to assure steady state. After that, the reactor washeated to the desired temperature (230° C.) and was run for 16 h withthe new parameters prior to measuring. Products were collected andanalysed during 20-26 h of synthesis time.

This catalyst exhibited a very good performance with a selectivity of81% for C5+ hydrocarbons, 10% for methane, 9% for light olefins and only0.3% for CO₂. Furthermore, its productivity was 0.45 g_(C5+)/g_(cata)/hwhere “g_(C5+)” refers to the weight of olefins having at least 5 carbonatoms and “g_(cata)” designates the weight of catalyst used. Thecalculated productivity, based on conversion, selectivity and CO flow,was 0.52 g_(C5+)/g_(cata)/h, i.e. slightly above the observed value andcomparable to the calculated productivity for a conventional catalystcomprising large cobalt crystallites (30-45 wt. %) supported on amicrometric titania carrier Finally, productivity calculated on cobaltmass basis was 2.62 g_(C5+)/g_(Co)/h for the catalyst of Example 1 and1.96 g_(C5+)/g_(Co)/h only for the conventional catalyst.

These results demonstrate that the novel core-shell nanoparticles ofthis invention allow the manufacture of a catalyst having, afterreduction, similar selectivities and higher productivities, in theFischer-Tropsch reaction, than conventional cobalt-based catalysts whichcontain much more cobalt.

The embodiments above are intended to be illustrative and not limiting.Additional embodiments may be within the claims. Although the presentinvention has been described with reference to particular embodiments,workers skilled in the art will recognize that changes may be made inform and detail without departing from the spirit and scope of theinvention.

Various modifications to the invention may be apparent to one of skillin the art upon reading this disclosure. For example, persons ofordinary skill in the relevant art will recognize that the variousfeatures described for the different embodiments of the invention can besuitably combined, un-combined, and re-combined with other features,alone, or in different combinations, within the spirit of the invention.Likewise, the various features described above should all be regarded asexample embodiments, rather than limitations to the scope or spirit ofthe invention. Therefore, the above is not contemplated to limit thescope of the present invention.

1. A method for the prepay of spherical core-shell particles, comprisingthe successive stops consisting of: (a) mixing a cobalt salt with (i)ammonium carbonate, bicarbonate or carbamate and (ii) ammonium hydroxidein water, so as to obtain an aqueous solution comprising a cobalt aminecomplex; (b) heating said aqueous solution to a temperature between 40and 90° C., either before, while or after adding under agitationparticles of alumina ha having a substantially spherical shape and anumber average diameter of less than 30 nm measured by TEM, so as todisintegrate alumina aggregates into primary particles and precipitatecobalt carbonate unto the surface of the alumina primary particles andto obtain core-shell particles having a core of alumina and shell ofcobalt carbonate; converting cobalt carbonate in the shell of saidcore-shell particles it to cobalt oxide.
 2. The method according toclaim 1, wherein the temperature in step (b) is between 50 and 70° C. 3.The method according to claim 1, wherein the cobalt salt represents from0.5 to 10 wt. %, preferably from 1 to 8 wt. %, and more preferably from3 to 7 wt. %, of the solution in stop (a).
 4. Core-shell particlesobtainable according to the method of claim 1, comprising a core ofalumina and a shell of cobalt oxide, wherein said core-shell particilesare spherical with a number average diameter, as measured by TEM, ofbetween 10 and 30 nm and preferably between 10 and 20 nm and the numberaverage thickness of the cobalt oxide shell, as measured by TEM, isbetween 1 and 5 nm and preferably between 1 and 3 nm.
 5. Core-shellparticles obtainable according to the method of claim 1, comprising acore of alumina and a shell of cobalt oxide, wherein the shell of cobaltoxide consists mainly of cobalt oxide nanoparticles with a numberaverage diameter as measured by TEM of 3 nm.
 6. A catalyst comprisingcore-shell particles as defined in claim 4, embedded in a carriercomprising nanometric particles of a carrier material having a numberaverage diameter as measured by TEM of below 100 nm.
 7. A method formanufacturing an activated catalyst, said method comprising the stepsof: mixing the core-shell nanoparticles of claim 4 with nanometricparticles of a carrier material having a number average diameter asmeasured by TEM of below 100 nm and water so as to obtain a slurry.homogenizing and drying said slurry to obtain a porous catalyst andoptionally shaping said catalyst, reducing said catalyst in order to atleast partially convert cobalt oxide into elemental cobalt.
 8. Acatalyst obtained according to the method of claim 7 for use in theFischer Tropsch process or in hydrogenation reactions such as theconversion of nitrobenzene to aniline or the conversion of fattynitriles to amines or the hydrogenation of aldehydes to thecorresponding alcohols.
 9. The method according to claim 1 wherein thecore-shell particles obtained at step (b) are recovered and dried priorto step (c) of conversion.